This invention relates to a fluorescent display device and a control electrode therefor, and more particularly to a fluorescent display device adapted to eliminate occurrence of electromagnetic interference (EMI) and a control electrode therefor.
In general, a fluorescent display device includes an envelope evacuated to a high vacuum, In which filamentary cathodes acting as an electron source, a mesh-like control electrode for accelerating and controlling electrons emitted from the cathodes and anodes for emitting light due to impingement of electrons thereon are arranged. Electrons emitted from the cathodes are permitted to travel through openings of a mesh of the control electrode and then impinge on the anodes, leading to luminescence of a phosphor of each of the anodes.
Currently, such a fluorescent display device as described above is used in a variety of fields while exhibiting characteristics peculiar to a display device of the self-luminescence type. For example, it is widely used as a vehicle mounted display device. Vehicle mounted fluorescent display devices include a fluorescent display device constructed so as to directly display a velocity of a vehicle, a rotational speed thereof and the like on a display panel and that constructed into a light source structure of the type of projecting a display pattern on a front glass of a vehicle to provide a driver with a reflected virtual image thereof. A display system by the latter fluorescent display device of the light source type of providing a driver with the reflected virtual image is called head up display (hereinafter also referred to "HUD"). The vehicle mounted fluorescent display device is generally required to exhibit increased luminance as compared with fluorescent display devices used in other fields in order to permit display to be positively observed. In particular, a fluorescent display device for HUD is required to exhibit luminance of a level 5 times as high as that of the vehicle mounted fluorescent display device because it is required to exhibit satisfactory visibility while overcoming a reduction in quantity of light due to reflection and eliminating affection by light reflected on a road surface.
Now, a conventional fluorescent display device for HUD will be described with reference to FIG. 3.
The fluorescent display device includes an envelope 3 constituted of an anode substrate 1 and a front substrate 2 arranged opposite to the anode substrate 1 while being spaced therefrom at a predetermined interval, as well as a side plate (not shown) arranged between both substrates 1 and 2 so as to extend along an outer periphery of the substrates and sealedly joined thereto. The anode substrate 1 constituting a part of the envelope 3 is formed thereon with wiring conductors 4, which are covered with an insulating layer 5. The insulating layer 5 is formed with through-holes 7 in a manner to positionally correspond to the wiring conductors 4. The through-holes 7 each have a conductive material 8 filled therein. Also, the insulating layer 5 is provided thereon with anode conductors 6 in a manner to positionally correspond to the through-holes 7 or cover the through-holes 7, so that the anode conductors 6 may be connected through the conductive materials 8 in the through-holes 7 to the wiring conductors 4, respectively. The anode conductors 6 each have a phosphor layer 9 deposited thereon, resulting in providing anodes 10. Further, the envelope 3 is provided therein with a mesh-like control electrode 11 in a manner to be positioned above the anodes 10. In addition, the envelope 3 has filamentary cathodes 12 stretchedly arranged therein in a manner to be positioned above the mesh-like control electrodes 11. The filamentary cathodes 12 act as an electron source.
The control electrode 11, as shown in FIG. 4, is constructed into a mesh-like structure which is formed of a wire of 15 to 35 .mu.m in diameter into a hexagonal shape so that six sides thereof each are 0.25 mm in length.
The fluorescent display device for HUD thus constructed, as described above, is required to exhibit increased luminance, resulting in being typically subject to static drive. On the contrary, dynamic drive of the fluorescent display device may be carried out at a duty ratio one half as large as that in the static drive.
However, operation of the fluorescent display device for display under the conditions that a cathode voltage is set at a rated input, the control electrode is driven at a duty ratio of 1/1 or 1/2 and the anodes are fed with a display signal causes EMI at a band of 350 MHz to 420 MHz to occur in a magnitude of 50 to 80 dB. For example, in the United states, the band is set for military purposes, therefore, the standards concerned define that EMI must be reduced to a level of 15 dB or less. Also in Japan, the standards concerned likewise demand to reduce EMI to a level of 15 dB or less.
The inventors considered that occurrence of such EMI is caused due to BK oscillation. BK oscillation means Barkhausen-Kurtz vibration and indicates electric vibration which occurs when a lattice of a triode is set at a positive voltage and an anode is set at a zero or negative voltage. It would be considered to be due to reciprocation of charges. BK oscillation will be more detailedly described with reference to FIG. 3. When anodes 10 are excited for luminescence or when the cathodes 12, control electrode 11 and anodes 10 each are kept at an ON-state, electrons emitted from the cathodes 12 are permitted to be accelerated through openings of the mesh-like control electrode 11 and then impinged on the anodes. However, when the anodes 10 are not driven or when the cathodes 12 and control electrode each are kept at a ON-state while keeping the anodes 10 at an OFF-state, electrons emitted from the cathodes 12 have nowhere to go and are repelled by the anodes 10, resulting in returning toward the cathodes 12 through the openings of the mesh-like control electrode 11. This would cause vibration of the electrons, leading to generation of a magnetic field, resulting in occurrence of EMI at a fixed frequency band.